The present invention relates to fluid flow control valves and particularly valves of the type controlling flow from a supply or source of pressurized fluid to an auxiliary outlet for connection to a fluid function circuit where a variable portion of the flow may be bypassed to a return passage to the supply. Bypass valves, as they are sometimes referred to, have found particular application in controlling flow of heated engine coolant to an on-board heat exchanger or heater core for temperature control in the passenger compartment of a motor vehicle. Typically, such valves are operated by manual cables, vacuum actuators or electric motor servo actuators. Bypass heater valves for automotive passenger compartment climate control applications have commonly employed a rotating throttle plate or butterfly for simplicity and relatively low manufacturing costs.
However, a simple butterfly-type valve has the characteristic that full flow is achieved when the valve is opened approximately one-fourth of its movement from the closed to the fully open position. Thus, accurate control of the flow requires precise positioning control of the butterfly by the actuating mechanism. This property of the butterfly valve has resulted in problems in maintaining the flow to the heater at the desired level with a servo mechanism in an automatic temperature control system for the passenger compartment.
Furthermore, butterfly-type water valves employed in a bypass valve configuration exhibit the characteristic that at nearly closed rotary positions of the butterfly, flow to the heater core does not occur and even reverse flow has been encountered in the region of the butterfly plate; and, the butterfly has been rendered ineffective in directing flow to the heater core channel. Accordingly, it has been desired to provide a way or means of preventing abnormal flow effects in a butterfly type bypass configured water valve and to improve the linearity of the flow control of the valve over the range of movement of the butterfly plate.
Referring to FIG. 15, a graphical plot of flow through the heater core outlet for an existing prior art by-pass valve versus the rotary position of the butterfly valve, when rotated from the closed position to the full flow position for flow to the heater core outlet; and, for flow upon return to the closed position is illustrated. It will be observed from FIG. 15 that the valves of the prior art produce no flow to the auxiliary or heater core circuit until the valve is opened substantially; and, upon reaching full flow to the heater core outlet there is a significant amount of hysteresis upon closing of the valve.